The present invention refers to a method for performing work in the ground or on rock, where impacts are dealt on an anvil by a hydraulically driven working piston, as well as to a hydraulic percussion device for performing such work in the ground or on rock.
The term work in the ground or on rock refers to drilling in the ground or in rock, in particular to impact drilling; among others, this also includes superposed drilling with an inner drill column and an outer drill column, as well as the operation of rock breaking devices wherein a working tool in the form of a chisel is driven into rock by percussion, so as to break the rock.
Hydraulic percussion devices are known that strike on the adapter end of a pipe column for drilling work or on the chisel of a rock breaking device. The efficiency of such a percussion device depends on the energy of the single impact and on the percussion rate. A high single impact energy is achieved if the working piston of the percussion device has a great mass. To accelerate such masses, high pressures are required. In practice, the mass of the working piston is several kilos and the piston stroke is 35 mm, for example. Typical piston rates are 7 to 11 m/sec. The achievable percussion rate is between 250 and 3,500 impacts/min. If the single impact energy is to be increased, it is common to increase the mass of the working piston, which, however, generally results in a decrease of the percussion rate.
From German Patent 43 43 589 C1, a fluid operated impact drill is known, wherein the working piston is controlled by a control piston and strikes on the adapter end of a drill column. In order to free the drill column when withdrawing the drill column, a back stroke piston is provided that strikes on a counter strike surface of the adapter end opposite the anvil. Here, the back stroke piston is activated only when the working piston is deactivated.
It is the object of the present invention to provide a method for performing ground or rock work and a hydraulic percussion device to achieve a higher efficiency of the percussion work, i.e. an increased drill advancement or a higher breaking efficiency (in rock breaking).
The present method and hydraulic percussion device provide for at least two working pistons striking in the same direction on an anvil. Thus, the anvil is acted upon by two working pistons, the impacts dealt by the working pistons preferably being offset in time. This results in an increased percussion rate without the single impact energy being lowered by a reduction of the piston mass.
Basically, the pistons are intended to strike the anvil at different times. The movements of the working pistons can be synchronized such that the working pistons are operated with mutually offset phases, so that two working pistons would be phase-shifted by 180xc2x0, for example. This means that one working piston performs the impact stroke while the other piston performs the return stroke. In another alternative, the two working pistons operate independent from each other and at different rates. Here, it is assumed that the impacts of the working pistons are normally offset in time and that the both working pistons happen to strike concurrently only at certain moments.
Another variant of the invention provides that the impacts from the working pistons are dealt synchronously, i.e. at the same time. In this case, the working pistons have to be operated at the same working rate and without mutual phase shift. It is also possible to provide a percussion device such that the working pistons can optionally be operated synchronously and asynchronously.
The invention provides for a high number of impacts (percussion rate), whereby the drill column is kept in constant movement (vibration) during drilling. Since most grounds contain an amount of grainy material that is caused to move by the high number of impacts, a very great drill feed is achieved in percussion drilling. Furthermore, the present method is adapted to prevent bouncing impacts that occur when an impact meets a shock wave traveling back along the drill column. Due to high number of impacts, the next impact is always performed already when the returning wave has not yet reached the rear end.
The present invention allows for numerous variants of control for the at least two working pistons. Both working pistons may be controlled separately and completely independent from each other. Alternatively, a control is conceivable where both working pistons are equal or a control, where one working piston functions as a master and the other functions as a slave.
In the context of the present invention, the impacts on the anvil are dealt by different working pistons. Preferably, the working pistons have substantially the same mass. That means that difference between the masses is 10% at most. However, the masses may differ more, yet the mass of the lighter working piston should not be less than two thirds, preferably not less than three quarters of the mass of the heavier working piston.